Device for aerating dispersions

ABSTRACT

A device and process for aerating dispersions, particularly for flotation of pulp suspensions, in a de-inking process where the pulp suspension containing dirt particles is sprayed into a tank together with air. The air is injected at a minimum of two successive points and mixed with the suspension.

BACKGROUND OF THE INVENTION

This invention relates generally to apparatus and processes for aeratingdispersions. More particularly, the present invention relates toapparatus and processes for aerating pulp suspensions during de-inking.

De-inking flotation is a mechanical process for removing impurities andink particles from pulp suspensions produced particularly in waste papertreatment. This process requires the generating of gas bubbles in theappropriate quantity and size distribution. Hydrophobic substances orsubstances to which ampholytics are added to make them hydrophobic, suchas ink particles or stickies, are carried to the surface of the liquidby the gas bubbles adhering to them and can be removed from the surfaceas scum. This is referred to as selective flotation because the pulp isdischarged with the accept due to its hydrophile nature. Processes ofthis type are known in numerous geometric modifications, for examplefrom DE 41 16 916 C2 or EP 0 211 834 B1, and have reached a hightechnical standard. Further, it has also proved successful to useself-priming injectors to generate gas bubbles and mix these with thepulp suspension. These injectors basically comprise a propulsive jetnozzle, a mixing or impulse exchange pipe, and a diffuser. Here, theliquid flow emerging from the propulsive jet nozzle according to theopen jet principle generates under pressure. As a result, gas is suckedin and mixed with the liquid as a result of the impulse exchange betweenliquid and gas in the mixing pipe. At the exit from the diffuser usedfor energy recovery a dispersion of pulp and bubbles is formed. Use ofthe known processes and injectors, however, has shown severaldisadvantages in selective flotation of pulp suspensions.

The suction effect of the known injectors in operation with pulpsuspensions is too weak and the bubble size distribution generated bythe injector known does not have the optimum design to meet therequirements of selective flotation.

SUMMARY OF THE INVENTION

The invention is, therefore, based on the task of designing an injectorwith greater suction effect and optimum bubble size distribution for usein de-inking flotation.

The process according to the invention is thus characterized by the gas,particularly air, being sucked in by the effect of the injector at aminimum of two successive points and mixed with the suspension. Due tosuction taking place in stages, the pulp can be loosened by the gas inthe first stage, thus achieving a better spread of the free jet in thesecond stage, resulting in improved suction effect and correspondingbubble generating, particularly with a reduction in the fine bubbleportion to avoid solids losses.

An advantageous further development of the invention is characterized bysome 20 to 95% of the entire quantity of gas, particularly air, suckedin being taken in the first stage. Since intake of the quantity of gas,particularly air, is divided over several suction points, more evenmixing of the suspension with the gas is obtained. This allows aspecific suitable bubble size to be set.

A favorable configuration of the invention is characterized by the gasand liquid flow obtained by suction and mixing being transferred in afree jet after the first stage. As a result, use of the kinetic energyof the jet, in particular, can be improved for renewed intake of gas.

A favorable further development of the invention is characterized by thegas or air loading of the pulp suspension directly after being sprayedin amounting to approximately 50-150%.

The invention also refers to a device for aerating dispersions,particularly a flotation device for de-inking pulp suspensions with aninjector, characterized by at least two suction points being arranged inseries in flow direction. Due to suction taking place in stages, thepulp can be loosened by the gas in the first stage, thus achieving abetter spread of the free jet in the second stage, resulting in improvedsuction effect and corresponding bubble generating, particularly with areduction in the fine bubble portion to avoid solids losses.

A favorable further development of the invention is characterized by theinjection channel widening after the first suction point. Thus, thekinetic energy of the jet can be put to good use in a favorable manner.

An advantageous further development of the invention is characterized bya panel being mounted at the end of the injector channel across the flowdirection. This panel acts as a radial diffuser to recover energy fromthe liquid jet.

An advantageous configuration of the invention is characterized by thepanel being mounted on a slant to the flow direction.

A favorable further development of the invention is characterized by thepanel containing internals for targeted guidance of the flow. As aresult, the injector can also be mounted in any desired position in theflotation cell.

A favorable configuration of the invention is characterized by a minimumof two injectors being mounted in parallel in the form of an injectorgroup. With this design it is also possible to handle large throughputsaccordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and its numerous objectsand advantages will become apparent to those skilled in the art byreference to the accompanying drawings in which:

FIG. 1 is a cross section view of a flotation unit having an aeratingdevice in accordance with the invention;

FIG. 2 is a cross section view of a second embodiment of an aeratingdevice in accordance with the invention;

FIG. 3 is a cross section view of a third embodiment of an aeratingdevice in accordance with the invention;

FIG. 4 is a cross section view of a fourth embodiment of an aeratingdevice in accordance with the invention;

FIG. 5 is a cross section view of a fifth embodiment of an aeratingdevice in accordance with the invention;

FIG. 6 is a graph comparing the bubble diameter distribution pattern ofan aerating device in accordance with the invention to that of aconventional aerating device;

FIG. 7 is a graph comparing the air loading, as a function of the Froudenumber, of an aerating device in accordance with the invention to thatof a conventional aerating device; and

FIG. 8 is a graph comparing the overall fiber loss, at a given airintake, of an aerating device in accordance with the invention to thatof a conventional aerating device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a diagram of the flotation unit in which the deviceaccording to the invention is installed. The flotation cell 9 is largelyfilled with suspension 8, on the surface of which scum 10 forms, whichcontains as large a portion as possible of impurities and ink particlesto be removed by flotation. This scum flows through a conduit 11 asoverflow U. The pulp suspension S enters the injector through thepropulsive jet nozzle 1. Due to the open jet principle, air is added atthe first suction point 2 and mixed into the pulp suspension in thefirst impulse exchange pipe 3. The pulp suspension loosened in this wayby the air bubbles sucks in more air 4 at the second suction point andthis air is mixed into the suspension in the second impulse exchangepipe 5. The air suction points are connected in this case to a pipeprotruding out of the suspension and into which air L enters at thesurface of the suspension. The dispersion 7 of bubbles and pulp leavesthe injector after passing through a radial diffuser 6 for energyrecovery purposes. The bubbles formed in this way adhere to thehydrophobic impurities and carry them to the surface. The suspensioncleaned by flotation leaves the flotation cell as accept pulp G.

FIG. 2 contains an alternative variant of an injector according to theinvention, where the gas intake fittings, for example, are mounted ondifferent sides. A significant difference to FIG. 1, however, is that aconically widening diffuser is installed after the second stage.

FIG. 3 shows a device according to the invention with a conically shapedfirst impulse exchange pipe 3, where a second propulsive jet nozzle isused analogous to the propulsive jet nozzle 1 so that high suctionefficiency is also achieved in the second stage.

FIG. 4 contains a design according to the invention in which three airintake points 2, 4, 12 are provided, with a diffuser shown after thethird impulse exchange pipe 13.

FIG. 5 shows a variant as injector group, where two injectors aremounted here in parallel beside each other. This arrangement comprises atop section, in which the propulsive jet nozzles 1 are mounted, a commonintermediate area into which the air intake fitting 2 leads, also ablock with impulse exchange pipes 3 operating in parallel. This block isconnected in turn to a common intermediate area into which the gasintake pipe 4 leads. This is adjoined by a common block where the secondimpulse exchange pipes 5 are mounted. Finally, both impulse exchangepipes 5 lead into a radial diffuser 6. It would also be possiblebasically to combine several injectors in an injector group of thiskind.

FIG. 6 now shows the bubble diameter distribution pattern of aconventional injector compared with that of an injector according to theinvention. This shows that the injector according to the inventioncontains significantly fewer bubbles with a diameter <0.5 mm than thestate-of-the-art injector. Here the reduction is approximately 50%.Unlike the conventional injector, however, the distribution spectrum isstill retained. Overall there are fewer solids (fiber) losses as aresult.

The suction effect of an injector is determined by the propulsive jetthroughput, the diameter of the propulsive jet nozzles, the liquid coverand the density of the propulsive jet. Suction characteristics of thistype are illustrated in FIG. 7. Here the air loading q_(G)/q_(L) isshown as a function of the Froude number. The illustration shows that,compared with conventional injectors, this air loading can be increasedsignificantly with the device according to the invention.

FIG. 8 contains a diagram of a flotation result at the same air intakecompared with that of a conventional injector. The diagram shows thatthe overall fibre loss could be reduced by approximately one third. Withthe present invention, however, it is possible to inject much more airand thus, also improve removal of impurities.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:
 1. An aerating device for use in a flotation unitfor de-inking pulp suspensions comprising a first injector defining aninjection channel, the injection channel having inlet and discharge endsdefining a flow direction and at least a first suction point and asecond suction point leading into the injection channel arranged inseries in the flow direction and further comprising a plurality ofimpulse exchange pipes, a one of the impulse exchange pipes beingdisposed downstream of each suction point in the flow direction. 2.Device according to claim 1, wherein the injection channeldiscontinuously widens after the first suction point.
 3. Deviceaccording to claim 1, further comprising a panel mounted at thedischarge end of the injection channel and extending across the flowdirection, the panel acting as a radial diffuser.
 4. Device according toclaim 3, wherein the panel is mounted on a slant to the flow direction.5. Device according to claim 3, wherein the panel includes at least oneinternal for targeted guidance of the flow.
 6. Device according to claim1, further comprising at least a second injector mounted in parallelwith the first injector, the first and second injectors defining aninjector group.